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Zorluk: ZorGeneral Internal Medicine

A 65-year-old man is brought to the emergency department due to profound weakness and lethargy. He has experienced large-volume, watery diarrhea for the past 5 days following a recent trip abroad. His medical history is significant for severe chronic obstructive pulmonary disease (COPD) requiring 2 L/min of home oxygen, and hypertension. Current medications include inhaled tiotropium, inhaled fluticasone-salmeterol, and lisinopril.

Temperature is 37.2°C (99.0°F), blood pressure is 98/60 mm Hg, pulse is 114/min, and respirations are 26/min. Physical examination shows dry mucous membranes, delayed capillary refill, and diffusely decreased breath sounds with a prolonged expiratory phase.

Laboratory studies show:
Serum sodium: 136 mEq/L
Serum potassium: 3.1 mEq/L
Serum chloride: 114 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 48 mg/dL
Serum creatinine: 1.9 mg/dL

Arterial blood gas (on room air):
pH: 7.24
PaCO2PaCO_2: 36 mm Hg
PaO2PaO_2: 62 mm Hg

Which of the following best characterizes this patient's acid-base disorder?

  1. Normal anion gap metabolic acidosis and concurrent respiratory acidosisCevap
  2. B
    Normal anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    High anion gap metabolic acidosis and concurrent respiratory acidosis
  4. D
    High anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Primary respiratory acidosis with compensatory metabolic alkalosis

Cevap

The patient has a normal anion gap metabolic acidosis and a concurrent respiratory acidosis.
The patient presents with an acidemic pH (7.24) and a low serum bicarbonate (HCO3−HCO_3^-) of 12 mEq/L, indicating a primary metabolic acidosis. The anion gap is calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-), which equals 136−(114+12)=10136 - (114 + 12) = 10 mEq/L. Because the normal anion gap is 8-12 mEq/L, this represents a normal anion gap metabolic acidosis, consistent with severe gastrointestinal bicarbonate loss from diarrhea. Next, Winter's formula (PaCO2=1.5×[HCO3−]+8±2PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2) must be used to assess the adequacy of respiratory compensation. For this patient, the expected PaCO2PaCO_2 is 1.5×12+8=26±21.5 \times 12 + 8 = 26 \pm 2 mm Hg (range 24-28 mm Hg). The actual measured PaCO2PaCO_2 is 36 mm Hg. Because the actual PaCO2PaCO_2 is significantly higher than the expected range, a concurrent, superimposed respiratory acidosis is present. The patient's severe underlying chronic obstructive pulmonary disease (COPD) prevents him from mounting the necessary hyperventilatory response to adequately compensate for the metabolic acidosis.

Adım Adım Çözüm

1
Assess the pH and primary disturbance.
The pH is 7.24 (acidemia) and the serum HCO3−HCO_3^- is 12 mEq/L (low).
An acidemic pH driven by a low bicarbonate level indicates a primary metabolic acidosis.
2
Calculate the Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=136−(114+12)=10AG = Na^+ - (Cl^- + HCO_3^-) = 136 - (114 + 12) = 10 mEq/L.
Determines whether the acidosis is driven by unmeasured anions (high AG) or bicarbonate loss (normal AG). A value of 10 mEq/L indicates a normal anion gap metabolic acidosis (NAGMA).
3
Apply Winter's formula to assess respiratory compensation.
Expected PaCO2=1.5×[HCO3−]+8±2=(1.5×12)+8=26±2PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = (1.5 \times 12) + 8 = 26 \pm 2 mm Hg (range 24-28 mm Hg).
Provides the physiological target for PaCO2PaCO_2 if the lungs are adequately compensating for the metabolic acidosis.
4
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2.
The actual PaCO2PaCO_2 is 36 mm Hg, which is significantly higher than the expected range of 24-28 mm Hg.
A higher than expected PaCO2PaCO_2 signifies an inability to adequately ventilate, revealing a superimposed, concurrent respiratory acidosis.

Anahtar Kavram

Mixed acid-base disorders and the application of Winter's formula
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Konular

General Internal Medicine

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